An important step in the manufacture of semiconductor chips and thin film circuitry is the etching of the different layers which make up the finished semiconductor chip or thin film circuit.
In the manufacture of thin film circuits, one method of etching has been to overlay the surface to be etched with a suitable mask and immerse the circuit and mask in a chemical solution which attacks the surface to be etched while leaving the mask intact. It has been difficult with the chemical etching processes presently known to achieve well-defined edges on the etched surfaces. The difficulty arises because the chemicals used for etching tend to undercut the mask, i.e. the wet chemical seeps under the mask and continues to attack the surface to be etched even under the masked area. A related difficulty which is encountered with certain materials is that the chemical etchant tends to eat through the surface to be etched and attack the substrate beneath. It is therefore very difficult to use wet chemical etching to achieve fine structures.
Etching of thin film circuits has also been done by a process sometimes called sputter etching. Typically a container such as a bell-jar is filled with an inert gas such as argon. In the container are placed an anode and a cathode, the latter of which is negative-biased relative to the former, e.g. by means of an applied RF signal. A surface to be etched is covered by a suitable mask of a material such as photoresist and is then placed on the cathode. When a negative bias is applied to the cathode, the inert gas in the region between the cathode and the anode is ionized and the positive ions are attracted toward the cathode. Those ions which strike the surface to be etched serve to knock atoms off the surface, thereby gradually etching through the material. In this process the photoresist mask is also etched but at a slower rate than the surface desired to be etched. Although this sputter etching process produces better defined edges than the chemical etching process, it is typically very slow, especially on TaN surfaces which are important in thin film work.
In the manufacture of semiconductor chips, another procedure (sometimes called plasma etching) is used, in which a container such as a bell-jar is filled with a gas such as CF.sub.4 whose constituent ions are chemically reactive. A surface to be etched is covered by a mask and inserted into the container along with the reactive gas. To etch the surface, an RF exciting coil around the container is activated to excite the CF.sub.4, thereby dissociating the CF.sub.4 and forming various positive and negative ions. The dissociated ions apparently then chemically interact with the surface to be etched producing various gases as a reaction product. As with the wet chemical etching process described above this type of plasma etching also results in undercutting of the mask areas so that it is difficult to achieve well-defined edges.